Integrated circuit designers have always sought the ideal semiconductor memory: a device that is randomly accessible, can be written or read very quickly, is non-volatile, but indefinitely alterable, and consumes little power. Magnetoresistive random access memory (MRAM) technology has been increasingly viewed as offering all these advantages.
A magnetic memory element has a structure which includes ferromagnetic layers separated by a non-magnetic barrier layer that forms a tunnel junction. Information can be stored as a digital “1” or a “0” as directions of magnetization vectors in these ferromagnetic layers. Magnetic vectors in one ferromagnetic layer are magnetically fixed or pinned, while the magnetic vectors of the other ferromagnetic layer are not fixed so that the magnetization direction is free to switch between “parallel” and “anti-parallel” states relative to the pinned layer. In response to parallel and anti-parallel states, the magnetic memory element represents two different resistance states, which are read by the memory circuit as either a “1” or a “0.” It is the detection of these resistance states for the different magnetic orientations that allows the MRAM to read binary information.
There are different array architectures that are used within MRAM technology to read memory cells. One architecture which is used is the so-called one transistor—one magnetic tunnel junction per cell (“1T-1MTJ”) architecture. This structure is based on a single access transistor for selecting a magnetic memory element for a read operation. Another architecture is the cross-point architecture, where a cell is selected and a read operation performed without using an access transistor. This type of system uses row and column lines set to a predetermined voltage levels to read a selected cell. Each system has its advantages and disadvantages. The cross-point system is somewhat slower in reading than the 1T-1MTJ system, as well as having a lower signal to noise ratio during a read operation; however, the cross-point array has the advantage that such arrays can be easily stacked within an integrated circuit for higher density. The 1T-1MTJ array is faster, has a better signal to noise ratio, but is less densely integrated than a cross-point array.
It would be desirable to have an MRAM read architecture that could utilize advantages from both the 1T-1MTJ and cross-point architectures, while minimizing the disadvantages of each.